A color image forming apparatus forms a color image by superimposing images of different colors on a sheet-like recording medium, for example a print paper. A color digital copying machine, for instance, reproduces an original color image by performing predetermined image processing on a color-separated image of an original document input through a scanner, forming a color-separated image of each color in an image forming section, and superimposing those images of different colors on a recording medium.
The color digital copying machine can faithfully reproduce the color image expressed in an original color document without distorting it, by faithfully reproducing the images of different colors and precisely superimposing those images of different colors on the recording medium.
For these reasons, to produce an image more faithfully to the original image, a color image forming apparatus has been developed lately which incorporates a process control unit for controlling image forming conditions in an image forming process to ensure faithful color reproduction in the image forming process for each color and which also incorporates a registration control unit for controlling the timing of image forming to precisely superimpose the images of different colors on a recording medium. Color copying machines have already been commercialised incorporating such a color image forming apparatus.
Japanese Laid-Open Patent Application No. 63-66578/1988 (Tokukaisho 63-66578) and Japanese Laid-Open Patent Application No. 63-279280/1988 (Tokukaisho 63-279280), among others, disclose technologies for a process control unit and registration control unit of this kind.
Japanese Laid-Open Patent Application No. 63-66578/1988 (Tokukaisho 63-66578) discusses control technologies for adjusting the image recording starting position on a transfer belt. Meanwhile, Japanese Laid-Open Patent Application No. 63-279280/1988 (Tokukaisho 63-27980) discusses control technologies for correcting the relative displacement of the images of different colors and for adjusting image density in a recording device for each color.
According to the technologies disclosed in those Patent Applications, the position and density of the pattern image formed on a transport belt is detected by a sensor, and then controlled in a suitable manner according to the signal output from the sensor.
A color image forming apparatus disclosed in Japanese Laid-Open Patent Application No. 63-66578/1988 (Tokukaisho 63-66578) is arranged so that a displacement of an image recording starting position is detected on a transport belt by forming, using a recording device, a pattern image on the transport belt at a predetermined timing and reading the pattern image with a CCD (Charge Coupled Device). Subsequently, an image writing forming timing is corrected in a recording device by an image writing timing correcting circuit, which acts as a registration control unit, according to the detected amount of displacement.
An image forming apparatus disclosed in Japanese Laid-Open Patent Application No. 63-279280/1988 (Tokukaisho 63-27980) is arranged so that a displacement of an image is detected by transferring a pattern image visualised by a recording device for each color onto a transfer belt and measuring an interval between the pattern images transferred onto the transfer belt. Subsequently, a writing timing is changed in a recording device by a CPU (Central Processing Unit), which acts as a registration control unit, according to the detected amount of displacement.
The above image forming apparatus is further arranged so that the density of the pattern image for image density detection transferred onto the transfer belt is measured by a photosensor, and the CPU controls toner supply in the recording device according to the measured density of the pattern image.
Nevertheless, faithful color image reproduction is not an easy task even to a color image forming apparatus for performing a process control and registration control of this kind. One reason for that is the problem in the precision in detecting a pattern image by nothing else but the sensor for detecting the pattern image. Conventionally, in order to improve the precision in detection, a sensor has been selected with good characteristics and improvements have been made on a circuit for processing detection signals.
However, such modifications of the sensor as such still fail to address the variation in the output of detection signals from the sensor caused by the variation in placement conditions, such as distance and angle, of the sensor with respect to the pattern image, and cannot avoid resulting in a poor precision in detection. For these reasons, in order to improve the precision in detection by the sensor, improvement should be made on the installment of the sensor in relation to the detected object (pattern image).
Moving back to the Patent Applications mentioned above to see how the sensor is installed, a CCD as a sensor is disposed opposite to a planar segment of a transport belt suspended by a plurality of rollers in Japanese Laid-Open Patent Application No. 63-66578/1988 (Tokukaisho 63-66578), whilst a photosensor is disposed opposite to a belt drive roller for suspending and supporting a transfer belt in Japanese Laid-Open Patent Application No. 63-279280/1988 (Tokukaisho 63-27980).
However, in the Patent Applications mentioned above, no consideration is given to the manner how the sensor is installed to improve the precision in detecting a pattern image by the sensor. Simply disposing a sensor for detecting a mark on the surface of a transport belt opposite thereto in the above manners cannot realise faithful color image reproduction.
If a sensor is simply disposed opposite to a planar segment of a transport belt suspended by a plurality of rollers as disclosed in Japanese Laid-Open Patent Application No. 63-66578/1988 (Tokukaisho 63-66578), the distance between the sensor and the transport belt inevitably changes due to the vibration of the suspended transport belt and also due to displacement in the installment of the sensor.
Meanwhile, if a photosensor is disposed opposite to a belt drive roller for suspending and supporting a transfer belt as disclosed in Japanese Laid-Open Patent Application No. 63-279280/1988 (Tokukaisho 63-27980), the pattern image is detected when it is on a surface of a segment, of the transport belt, that has curved due to the round surface of the roller for the transport belt. Therefore the inclination of the optic axis of the sensor to the surface of the transport belt tends to vary, and the sensor output tends to change.
The rotation axis of the roller is eccentric in some cases, causing the roller to rotate elliptically rather than rotate circularly. As a result, the distance between the sensor and transport belt changes according to the location of the roller.
As illustrated in the above description, if no consideration is given to the manner how the sensor is disposed, the sensor output changes due to various reasons.